1. Field of the Invention
The present invention relates generally to a plasma process for treating material, and more particularly to a process for fabricating Schottky barriers and ohmic contacts in devices normally employed within the semiconductor electronics industry.
2. Description of Prior Art
Examples of patents which relate to formation of Schottky barriers by use of a plasma are U.S. Pat. No. 3,795,557, issued Mar. 5, 1974 to A. Jacob, and U.S. Pat. No. 3,879,597 issued Apr. 22, 1975 to Bersin et al; the disclosures of these two patents are incorporated herein by reference.
Metal-semiconductor rectifying devices have been investigated since the late 1800's. Sometime thereafter, around the turn of the century, a point-contact rectifier (metal-semiconductor junction device) may have found practical utility as evidenced by U.S. Pat. No. 775,840 issued in 1904 to J. C. Bose. In the late 1930's W. Schottky continued to study the metal-semiconductor potential barrier, and the suggestion that this potential barrier could arise from arrangement of space charges within the semiconductor, not necessarily requiring presence of an adjacent chemical layer, is attributed to him. This development continued through more recent history, and the metal-semiconductor junction rectifier presently employed in electronic circuit applications is commonly called a Schottky diode or Schottky transistor.
The Schottky barrier contact or interface, as noted, is a rectifying metal-semiconductor junction. Such Schottky barrier contacts utilize the Schottky effect based upon rectification characteristics exhibited by well known metal-semiconductor interfaces. Generally, the electrical characteristics of these contacts depend upon what is termed the "work function" of the metal, as well as the electron affinity in the semiconductor material. The work function is generally defined as the minimum energy necessary for an electron to have in order to escape into vacuum from initial energy at the Fermi level, in a metal-vacuum system. In other words, in a metal, the Fermi level is an energy reference level from which an electron is removed to the free state (vacuum electron) by an amount of energy equal to the work function.
High frequency response of these Schottky contacts or diodes is good, the positive results flowing from conduction phenomena which occurs under forward bias, caused primarily by majority carriers falling from the semiconductor into the metal. Accordingly, the otherwise frequency-limiting effect of minority carrier storage tends to be minimized. High frequency response of Schottky barrier diodes makes them quite useful in various high frequency applications, such as high speed logic and memory circuits, microwave applications, etc. The foregoing is presented as background in the history and application of the resulting semiconductor device, the device itself being prepared by the chemical/physical process of the present invention, the background of which follows.
In most, if not all, types of integrated circuits and discrete devices, it has been usually necessary to make ohmic contacts to various regions. For example, in Schottky-clamped-integrated circuits, and those employing Schottky barriers as active devices, whether integrated or in discrete form, it had been generally necessary not only to make ohmic contacts to several regions, but also to fabricate good Schottky barrier contacts. It is generally preferred to use the same metallization for both ohmic contacts and Schottky barriers in a particular device. Although one is not limited to this constraint, it is generally preferred from a production point of view.
In the prior art or conventional technology, when aluminum and/or its alloys with silicon and copper are used for metallization, a problem of nascent oxide (oxide which comes into being, or forms, or develops over bare silicon regions) is solved to some extent by heat treatments in a neutral ambient environment at temperatures between 450.degree. C. and 550.degree. C. Under appropriate processing conditions, aluminum atoms can move through the nascent oxide layer and make intimate contact with the silicon regions, yielding good Schottky barriers and ohmic contacts. Metals other than aluminum can yield poor results.
If a metal-silicide, such as platinum-silicide, palladium-silicide or rhodium silicide, is used to form the Schottky barriers and ohmic contacts, the nascent oxide layer on the silicon substrate can cause problems. The platinum-silicide is usually preferred over the others because its barrier is the largest on N-type silicon and because of its superior reliability/performance characteristics. In such metallization schemes other metal layers like titanium-platinum-gold (beam lead technology) and titanium-tungsten-aluminum are deposited on these metal-silicide layers. These metal-silicide devices are more reliable than those devices using aluminum and its alloys only.
In prior art platinum-silicide processes, the nascent oxide on silicon regions is inevitably present during the processing of the silicon wafers. One prior art solution was to use chemical solution of hydrofluoric acid to etch away oxide on the silicon surface; however, as soon as the wafers are rinsed in water, an oxide layer is again immediately formed. Thickness of this nascent oxide layer is about 25 Angstroms, (one Angstrom=10.sup.-10 meters) or greater, depending on processing conditions. This thickness is enough to prevent platinum from reacting with silicon to form a good, uniform layer of platinum silicide when the wafers are heat treated in a neutral ambient, even at 650.degree. C. This is a problem of the prior art.
Another prior art or presently used process to remove nascent oxide from silicon regions where platinum-silicide ohmic contacts or Schottky barriers are to be formed, is to do in-situ sputter-etching of the silicon wafers prior to platinum deposition in the same sputtering system. Sputter-etching is basically an ionic bombardment of the surface to be cleaned off. After the usual ambient pressure pump-downs in the sputtering system, back-filling with an inert gas like dry argon to a pressure of about 20-40 microns, and applying radio frequency (or negative d.c.) high voltage to the electrode on which the silicon wafers are placed, an argon plasma is obtained. Depending on the voltage/wattage of the d.c./r.f. power, and its duration, a certain thickness of oxide, silicon and/or other substances present on the wafer surface are removed. Sputter-etching, in this case, is a bombardment of the surface of silicon with highly energized ions of argon, thus removing these impurities from the surface of the silicon. Reference to U.S. Pat. Nos. 3,737,743 and 3,855,612 will provide further information on sputter-etching, as it relates to Schottky barrier devices.
The thickness of silicon oxide expected to be removed in this sputter-etching step is in excess of 100 Angstroms. Usually, more etching than that which is necessary is practiced in the processing to assure complete removal of the nascent oxide. This is done because an incomplete removal of the nascent oxide gives patchy or poor silicide formation, which in turn results in bad ohmic contacts and bad Schottky barriers.
However, there are problems also associated with this sputter-etching prior art process. If the sputter-etching is insufficient to remove all nascent oxide contaminants, there are problems with ohmic contacts and Schottky barriers since this imperfect process permits random variation such as increases in resistive values of the ohmic contacts. In turn, this permits other problems such as voltage and power loss at these increased-resistivity ohmic contacts with accompanying unpredictable device performance and reliability problems. Further, random variation such as decreases of reistivity of Schottky barrier potential heights permits higher leakage currents in Schottky devices as well as high-field edge-effects, with accompanying unpredictable device performance and again reliability problems. Finally, for this situation of insufficient sputter-etching, a problem of lifting of beam leads and contacts from the ohmic contact and Schottky barrier regions can occur.
But, suppose the sputter-etching were excessive, rather than inadequate. If excessive sputter-etching of the silicon wafers is done to ensure complete removal of nascent oxide layer, and to ensure good platinum silicide formation then other problems can occur as follows: First, excessive sputter-etching can remove an inordinate large amount of silicon oxide, in particular the phosphorus doped silicon oxide normally covering the emitter and various cross-under n-type regions. If this sputter-etching is not controlled properly, then the thin emitter glass (silicon) gives rise to low breakdown voltage and higher leakage and shorting problems. Next, if the oxide films have pinholes, excessive sputter-etching enlarges these pinholes, and can cause a short circuit.
Yet another problem which can occur during the prior art sputter-etching process is that if water vapor is present in the sputtering system, then an oxide layer can be created in this cycle due to reactive sputtering. Therefore, instead of removing the nascent oxide layer from silicon regions, an additional oxide layer is created- The water vapor in the system can come from inappropriate drying of the wafers prior to loading them into the sputter-etching system, and/or it can be obtained from other sources like argon gas, vacuum system and jigs used in the system. The mechanism of the formation of an oxide layer in this step is due to the reaction of the oxygen and/or hydroxide ion in the prior art ionized argon plasma with silicon. Therefore, instead of improving the silicon surface to give a good ohmic contact and a good Schottky barrier, the silicon surface is oxidized further, yielding bad results.
The present invention provides a solution to this multiplicity of problems of the prior art. The present invention employs and provides an improved process for manufacture of metal-semiconductor interfaces including but not limited to platinum-silicide interfaces, Schottky barrier potential junctions, and ohmic contacts, which improved process eliminates the sputter-etching step and thereby eliminates all of the above-detailed problems associated with the presently used and prior art sputter-etching technique. The present invention employs a novel plasma process to be described in detail hereinbelow.